A silver halide emulsion used for a silver halide photographic material is usually chemically sensitized with various chemical sensitizers to obtain a desired sensitivity or gradation. Examples of the chemical sensitizers include a sulfur sensitizer, a selenium sensitizer, a tellurium sensitizer, a noble metal (such as gold) sensitizer, a reduction sensitizer and a combination thereof.
An improvement of the silver halide photographic material has increasingly been demanded for several years. The recent photographic material requires very high sensitivity. Further, an improvement of the graininess and the sharpness has been required with respect to the formed image. A rapid image forming process such as a quick development process is also required. The sensitization and the sensitizers have been improved to meet these requirements.
Sulfur sensitization has most frequently been used in the silver halide photography. Numerous sulfur sensitizers have been known and used in silver halide photographic materials.
On the other hand, the sensitizing effect of selenium sensitization is higher than the effect of the sulfur sensitization. Accordingly, various selenium sensitizers have been proposed in place of the sulfur sensitizers. However, the proposed selenium sensitizers have a tendency that fogs easily occur in the image and gradation of the image is softened. Therefore, the selenium sensitizers should be further improved to be used in place of the conventional sulfur sensitizers. U.S. Pat. No. 3,297,447 discloses selenourea derivatives (e.g., dimethylselenourea) as improved selenium sensitizers. However, the improvement is still insufficient, and a rather dense fog is observed in the image (cf., Tables 1-5 in Examples of the present specification).
The sulfur sensitization and the selenium sensitization are usually used in combination with a gold sensitization. The combinations of the sensitizations remarkably improve the sensitivity of the photographic material. However, the combinations also increase the degree of fog in the image. The fog in the gold-selenium sensitization is more remarkable than that in the gold-sulfur sensitization. Accordingly, the fog caused by a selenium sensitizer should be reduced particularly in the case that the selenium sensitizer is used in combination with a gold sensitizer.
Further, the selenium sensitizers are usually not stable compounds. The sensitivity of a silver halide photographic material sensitized with a selenium sensitizer is sometime changed while the material is preserved. Therefore, a stable selenium sensitizer has also been required.